How genes and genomes are organised

A cell holds an enormous length of DNA, so it must be packed tightly.

In prokaryotes, there is usually one circular, double-stranded DNA molecule, in a region called the nucleoid, plus small extra rings, plasmids. It is compacted by supercoiling (twisting on itself), mostly negative supercoiling, helped by histone-like proteins.

In eukaryotes, DNA is packed in stages:

• DNA (2 nm wide) wraps around a core of eight histone proteins (an octamer: two each of H2A, H2B, H3 and H4), forming a nucleosome. About 146 base pairs wrap each core; with the linker DNA between beads, about 200 base pairs. Histone H1 binds the linker.
• The nucleosomes look like beads on a string (10 nm). They coil into a 30 nm fibre.
• The fibre forms loops on a protein scaffold (300 nm).
• At metaphase, the chromosome is about 700 nm wide.

Genomes grow larger and more complex from viruses to bacteria to eukaryotes. The human genome is about 3,000 Mb (3 billion base pairs) with more than 20,000 genes, but only about 2% codes for proteins; much of the rest is repeated sequence.

Eukaryotic genes are often split: coding pieces (exons) are interrupted by non-coding pieces (introns). The introns are removed from the first RNA copy and the exons joined, as in the beta-globin gene. Mitochondria and chloroplasts have their own small circular DNA, many copies each, inherited mostly through the mother.